This invention relates to a tone signal generation device and, more particularly, to a device capable of generating a tone signal of a desired tone color by performing a tone generating operation (computation) such as a frequency modulation operation and amplitude modulation operation. More particularly, the invention relates to a tone signal generation device in which a plurality of tone generation channels are provided for enabling simultaneous sounding of plural tones and which is capable of changing the maximum number of tones simultaneously produced depending upon a tone or tones to be produced.
A basic system for generating a tone signal by using a frequency modulation (hereinafter abbreviated as FM) operation in the audio frequency range is disclosed in the U.S. Pat. No. 4,018,121. A basic system for generating a tone signal by using an amplitude modulation (hereinafter abbreviated as AM) operation in the audio range is disclosed in Japanese Patent Publication No. 29519/1983. Known also in the art is an electronic musical instrument which includes a limited number of tone generation channels wherein the sounding of a tone for a depressed key is assigned to any of the tone generation channels whereby different tones corresponding in number to the number of the tone generation channels can be simultaneously produced (e.g., the U.S. Pat. No. 3,882,751).
It is also known to employ one of the above described basic operation systems (FM or AM) as a tone generation system in each of such a plurality of tone generation channels and to sound plural tones generated by such operation systems simultaneously. In such prior art electronic musical instruments, the number of tone generation channels is fixed and it is not possible to change this number depending upon the situation.
In a tone generation system employing the above described operation systems, one or more basic operation units (or operation channels) are used in one tone generation channel and operation parameters in each operation unit are suitably selected to generate a tone signal of a desired tone color. In this case, the more the operation units are used, the more various and complex tone color control one can realize. Therefore, the number of basic operation units per one tone generation channel should preferably be large if the tone color or quality of a tone to be achieved is important. Further, there is a case in which a sufficient number of basic operation units are required depending upon the tone color or performance mode selected. On the other hand, there is also a case in which the number of the operation units per one tone generation channel need not be large whereas the number of tones which can be sounded simultaneously should be large. For satisfying the former requirement, the number of the operation units (operation channels) per one tone generation channel must be sufficiently large whereas for satisfying the latter requirement, the number of the tone generation channels must be large. Therefore, there arises the problem that it requires a large and costly device to satisfy both these requirements. Although a plurality of operation units can be realized by using a single basic operation circuit on a time shared basis, the increase in the number of operation units obliges an increase in the time division clock rate which result in a rise in the manufacturing cost. Further, aside from such problems, many unused operation units (operation channels) will be wasted in such a device having a large number of tone generation channels and a large number of operation units when a tone color or a performance mode which does not required a large number of operation units per one tone generation channel has been selected.